Since phosphazene compounds, especially cyclic and linear oligophosphazene compounds contain phosphorus atoms, they are expected to show a high refractive index when used as an optical material. Thus some documents suggest application to optical materials having high refractive indices. For example, Macromolecules 1997, 30, 4179-4183 describes that a linear polyphosphazene compound including more than 1,000 phosphazene units (—N═P—) in which phenyl groups, naphthoxy groups, halogenated phenyl groups, halogenated naphthyl groups, derivatives thereof or the like are bonded to the phosphorus atoms of the phosphazene skeleton can be used as an optical material. This polyphosphazene compound exhibits a high refractive index of more than 1.6, but exhibits an Abbe number of about 25 at most because of the aromatic rings in the structure, thus causing large optical dispersion. Abbe number is an index of optical dispersion. The larger the numerical value, the smaller the optical dispersion becomes.
Further, a Japanese Unexamined Patent Publication No. 7-198967 describes a light transmission body in which a transparent clad material is filled with a liquid of an oligophosphazene compound. The oligophosphazene compound used herein is a compound in which alkoxyl groups, aryloxy groups, amino groups, derivatives thereof or the like are bonded to the phosphorus atoms of the phosphazene skeleton, and exhibits high transparency through a wide wavelength range including the ultraviolet region, the visible region and the infrared region. Since this oligophosphazene compound is a liquid, it can be used as an optical material to be filled in the above clad material, but is impractical to be used as a material for optical molding such as an optical lens.
An object of the present invention is to realize an oligophosphazene compound capable of forming a transparent polymer which has a high refractive index and a high Abbe number, and thus is usable as an optical material.